High-density disc reflection spectroscopy of low-mass active galactic nuclei

• Published in: arXiv.org 2022-04
• Main Authors: Mallick, L, Fabian, A C, García, J A, Tomsick, J A, Parker, M L, Dauser, T, Wilkins, D R, De Marco, B, Steiner, J F, Connors, R M T, Mastroserio, G, Markowitz, A G, Pinto, C, Alston, W N, Lohfink, A M, Gandhi, P
• Format: Article
• Language: English
• Subjects: Accretion disks; Active galactic nuclei; Black holes; Broadband; Compressive strength; Density; Deposition; Radiation; Radiation pressure; Soft x rays; Spectrum analysis; X ray spectra; X-ray astronomy
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2203.04522

The standard alpha-disc model predicts an anti-correlation between the density of the inner accretion disc and the black hole mass times square of the accretion rate, as seen in higher mass (\(M_{\rm BH}>10^{6} M_{\odot}\)) active galactic nuclei (AGNs). In this work, we test the predictions of the alpha-disc model and study the properties of the inner accretion flow for the low-mass end (\(M_{\rm BH}\approx 10^{5-6}M_{\odot}\)) of AGNs. We utilize a new high-density disc reflection model where the density parameter varies from \(n_{\rm e}=10^{15}\) to \(10^{20}\) cm\(^{-3}\) and apply it to the broadband X-ray (0.3-10 keV) spectra of the low-mass AGN sample. The sources span a wide range of Eddington fractions and are consistent with being sub-Eddington or near-Eddington. The X-ray spectra reveal a soft X-ray excess below \(\sim 1.5\) keV which is well modeled by high-density reflection from an ionized accretion disc of density \(n_{\rm e}\sim 10^{18}\) cm\(^{-3}\) on average. The results suggest a radiation pressure-dominated disc with an average of 70% fraction of the disc power transferred to the corona, consistent with that observed in higher mass AGNs. We show that the disc density higher than \(10^{15}\) cm\(^{-3}\) can result from the radiation pressure compression when the disc surface does not hold a strong magnetic pressure gradient. We find tentative evidence for a drop in black hole spin at low-mass regimes.